Are there any guidelines for appropriate use of the Black HUD UI in OS X?

Question

Does anyone know what the HIG are saying about the black HUD UI in OS X?

Answer 1

See the Transparent Panels section of Apple's Human Interface Guidelines.

Answer 2

Basically HIG say that black HUD UI is named a transparent panel.

Its role is the same as a standard panel, i.e., an auxiliary window that offers some controls related to the current document.

Its look is as discrete as possible, in particular in highly visual applications.